1. Field of the Invention
This invention relates to an electrodeposition process which comprises electrodepositing onto a conducting substrate a coating composition comprising a self-crosslinking polymeric resin which has an average of at least two pendent hydroxy functional groups and an average of at least two pendent ester groups of the general formula: ##STR2## wherein R.sup.1 and R.sup.2 are independently hydrogen, alkyl, substituted alkyl or aryl groups; n is from about 1 to about 5; X is nitrogen, sulfur or phosphorus; and when X is nitrogen or phosphorus, R.sup.3 is hydrogen, lower alkyl, or an aryl group; R.sup.4 is lower alkyl or an aryl group; and when X is sulfur, R.sup.4 is nothing and R.sup.3 is either hydrogen, lower alkyl or aryl.
2. Description of the Art
Methods of crosslinking hydroxy functional polymers or polymeric polyols with esters are known. For example, U.S. Pat. No. 4,423,167 teaches coating compositions formed by transesterifying a polymeric polyol with a polyester crosslinking agent having at least two beta-alkoxyester groups per molecule. Similarly, U.S. Pat. No. 4,440,612 teaches a method of coating a conductive substrate using the composition of U.S. Pat. No. 4,423,167. U.S. Pat. No. 4,489,182 teaches coating compositions formed by transesterifying a polymeric polyol with a polyester crosslinking agent having at least two gamma and/or delta-hydroxyester groups per molecule. U.S. Pat. No. 4,511,447 teaches a method of coating a conductive substrate using the composition of U.S. Pat. No. 4,489,182. U.S. Pat. No. 4,423,169 teaches coating compositions formed by transesterifying a polymeric polyol with a polyester crosslinking agent having at least two beta-carboxyalkyl and/or gamma-carboxyalkyl ester groups per molecule. U.K. Patent Application No. GB 2 102 430 A teaches coating compositions formed by transesterifying a polymeric polyol with a polyester crosslinking agent having at least two ester groups per molecule, selected from the group consisting of beta-alkoxyester, beta-carboxyalkyl, beta-amido, gamma-hydroxy, gamma-carboxyalkyl, and delta-hydroxy groups. European Patent Application No. 0 012 463 A1 teaches thermosetting resinous binders useful as coating compositions comprising a hydroxy-functional polymeric resin and a polyester crosslinking agent having at least two beta-hydroxyl ester groups per molecule. U.S. Pat. No. 4,427,805 teaches thermosetting binder compositions comprising a hydroxy-functional polymeric resin and a polyester crosslinking agent having at least two beta-hydroxyl ester groups per molecule. U.S. Pat. No. 4,491,611 teaches catalytically self-crosslinking resinous binders prepared by addition reaction of a primary amine to a beta-hydroxyalkyl unsaturated ester followed by reaction of this amino adduct with an epoxy resin to produce a resin having both hydroxy functionality and beta-hydroxyalkyl ester groups.
In each of the above prior art patents, the crosslinking mechanism is promoted by an activation of the reactive ester group. This is accomplished through interaction with the available electron pair on the neighboring oxygen atoms from the beta- or gamma-hydroxy or alkoxy group. The degree of activation is directly proportional to the degree of the availability of the electron pair, i.e. nucleophilicity, from the activating oxygen. The nucleophilicity of oxygen in the prior art is low compared to the nucleophilicity of nitrogen, sulfur or phosphorus, consequently, the magnitude of activation provided by oxygen is lower, and the result is the need for generally higher crosslinking temperatures than those needed by the present invention.
The present invention relates to an electrodeposition process which comprises electrodepositing onto a conducting substrate a coating composition comprising a self-crosslinking polymeric resin which has an average of at least two pendent hydroxy functional groups and an average of at least two pendent ester groups of the general formula: ##STR3## wherein R.sup.1 and R.sup.2 are independently hydrogen, alkyl, substituted alkyl or aryl groups; n is from about 1 to about 5; X is nitrogen, sulfur or phosphorus; and when X is nitrogen or phosphorus, R.sup.3 is hydrogen, lower alkyl, or an aryl group; R.sup.4 is lower alkyl or an aryl group; and when X is sulfur, R.sup.4 is nothing and R.sup.3 is either hydrogen, lower alkyl or aryl.